In order for an amplifier (e.g., a class AB amplifier) to operate according to specified design criteria, the quiescent bias current of an MOS device, including a lateral diffused MOS (LDMOS) power transistor, used in the amplifier must remain substantially constant over temperature and/or process variations. Unfortunately, this bias current is largely dependent upon, among other parameters, a threshold voltage of the MOS device, which exhibits a strong temperature dependence. Additionally, since the threshold voltage of the MOS device is significantly affected by variations in semiconductor process parameters, the threshold voltage typically varies widely from device to device.
Conventional amplifiers are known to employ external bias generators having temperature compensation circuits for providing a relatively temperature-independent bias current for biasing a discrete MOS device used in the amplifiers. For example, it is well-known to use a bandgap reference generator, which is essentially an operational amplifier in conjunction with a temperature tracking diode for detecting temperature variations. Other known methodologies for tracking temperature may include the use of software lookup tables. However, since these bias circuits are external to the packaged MOS device, such conventional compensation methodologies, which often rely on sensing an ambient air temperature or a case temperature of the device, are not able to accurately track the temperature and/or process characteristics of the MOS device itself. Moreover, these additional circuits and/or methodologies are often very complex and may require manual bias current adjustments (e.g., in order to compensate for process variations) for each amplifier utilized, therefore making such circuit configurations costly to implement.
There exists a need, therefore, for techniques capable of accurately compensating for variations in a bias condition of an MOS device resulting, at least in part, from temperature and/or process variations of the device, without suffering from one or more of the problems exhibited by conventional circuits and methodologies.